1. Field of the Invention
This invention relates to a method of polarizing an electric motor for use in a compressor, etc.
2. Description of the Related Art
Recent air conditioners have a compressor of a variable capacity type for performing an energy-saving operation. Compressors of this type are equipped with a speed-variable type electric motor driven by the output of an inverter.
Some of the above compressors have a stator with a plurality of phase windings and a rotator with a plurality of permanent magnets. The rotator rotates as a result of interaction between rotary magnetic fields created by the phase windings and magnetic field created by the permanent magnets.
Each of the permanent magnets is molded by stiffening magnetic particles, and then mounted on the rotator. In this stage, the permanent magnets are not polarized. A polarization treatment is performed after rotator components are assembled. Specifically, the permanent magnets are polarized by connecting the phase windings of the stator to polarizing power source and supplying a polarizing current to the phase windings.
At the time of polarization, it is necessary to align the central points of the permanent magnets with lines of magnetic force created by the polarizing current. If such an alignment is not performed at the time of polarization, each permanent magnet may not be sufficiently polarized, or the magnetic poles of the magnet may be displaced from predetermined positions.
When the magnet is not sufficiently polarized, the efficiency of the electric motor is reduced.
On the other hand, when the magnetic poles are displaced from predetermined positions, the position of the rotator cannot be accurately detected, which may cause loss of synchronism in the motor during operation or render the motor inoperable. As a result, an abnormal current may flow through each phase winding, and hence circuit elements of an inverter employed on the driving side be broken.
To avoid the above, positioning is performed beforehand so that the central points of the permanent magnets are aligned with lines of magnetic force created by the phase windings. In the conventional method, however, the positioning is performed based on eye-estimation of a worker, and therefore cannot be done accurately.